The Air Force has cleared 60% of its F-15 fleet to resume operations, ending a two-month standdown that resulted from structural problems in the aging jets. Those problems were blamed for the crash of a Missouri Air National Guard F-15 in November that literally broke apart during routine training maneuvers.
Tuesday's return-to-flight order was issued by General John Corley, Commander of Air Combat Command, and covers about 265 F-15s, models A through D. That represents 60% of the service's air superiority F-15s. Corley made his decision after reviewing findings of the Missouri accident investigation board, and a briefing from structural experts representing Boeing, which builds the F-15.
However, Air Force Times reports that the service's remaining A/D model F-15s--a total of 175 aircraft--will remain grounded for at least another month. Their return to flight hinges on a study by the Warner-Robins Air Logistics Center, to determine their air-worthiness.
Analysis of the Missouri crash--and the on-going study--have focused on fatigue cracks in the F-15s forward longerons, which help ensure the aircraft's structural integrity. Cracks were found in the Eagle that went down in Missouri, and a subsequent, fleet-wide inspection turned up similar problems in nine other jets, assigned to both active duty and Air National Guard (ANG) units.
The Air Force's 224 multi-role F-15E Strike Eagles are not affected by the problem. After a brief standdown in November, they returned to operational service.
According to an Air Force statement, the F-15s which will remain grounded are "scattered across several squadrons." Those units were not identified.
Among units that were cleared to fly, operations resumed quickly. The 71st Fighter Squadron, one of the Air Force's original F-15 units, returned to the skies this morning with the launch of six jets at Langley AFB, Virginia. Four other Eagles flew this afternoon; the squadron is expected to resume a full flying schedule in the coming days, trying to complete training that was delayed by the standdown.
The resumption of F-15 operations at Langley was noted by the local paper, the Newport News Daily Press. However, the photo that accompanied the on-line story showed F-15E Strike Eagles from Seymour Johnson AFB, North Carolina--not the air superiority C and D models flown by the 71st. Oops.
Return of some F-15s to operational status will give military commanders greater flexibilty in accomplishing various missions, including air defense of the United States. The recent grounding affected several air defense units that fly the Eagle, forcing Air Force F-16 squadrons and Canadian CF-18s to assume greater responsibilities.
The F-15 standdown also affected Eagles operated by key U.S. allies, including Israel, Saudi Arabia and Japan. It was unclear when those nations would clear their jets for flight. However, foreign F-15 customers have followed the U.S. lead in grounding and inspecting the aircraft.
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Between these and the horrid progress of the JSF, the military needs more reliable fighters now. Check out this article on the JSF - Lockheed's really done it this time.
MANUFACTURING FLAW FOUND IN STOVL JOINT STRIKE FIGHTER LIFT FAN ROTOR
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Date: January 4, 2008
Program officials have discovered a manufacturing defect in the lift fan rotor of the short take-off vertical landing F-35 Joint Strike Fighter test aircraft that will require the installation of new components before flight and ground trials of the stealth aircraft begin later this year.
The lift fan rotor is part of a complex propulsion system that allows the next-generation fighter to hover, a technology that is essential to the capabilities of the JSF STOVL variant, the F-35B.
Inside the Air Force has learned that during ground tests this fall, the JSF program office discovered problems in a key part of the lift fan built by Pratt & Whitney and installed in STOVL aircraft BF-1, flaws that will require the lift fan rotor to be replaced.
Bill Gostic, vice president of the F135 engine program for Pratt & Whitney, said in a Jan. 3 interview that during manufacturing, a process he said is “not trivial,” some of the fan blades welded to a fan disk were problematic.
“Sometimes in the early stages in your manufacturing processes,” Gostic said, “you have some dimensional non-conformances. We had one in this case.”
Defense Department officials said they are convinced that this problem reflects a manufacturing glitch, not a more significant design flaw, and that correcting the defect will not force a delay to the flight test plan or any other part of the program.
“We were on a schedule where [JSF prime contractor] Lockheed Martin really needed a lift fan, and they were not planning on operating the lift fan, so, we went ahead and delivered a full-up lift fan assembly with this non-conforming, first stage lift fan rotor . . . with the plan that we would go and replace that rotor before we go actually operate the lift fan in any sort of flight test program,” Gostic said.
At that time, Pratt & Whitney did not have another rotor to swap out with the non-conforming one, he said. The non-conforming rotor currently in the jet is being used primarily for lift fan fit checks.
The “current lift fan will be swapped for another lift fan,” said Cheryl Limrick, a spokeswoman for the JSF joint program office, in a Dec. 20 e-mail. “The current plan for STOVL engine qualification and flight test engine delivery does not impact the JSF program schedule and or first flight of the STOVL aircraft.”
Flight testing on the STOVL F-35 is slated to begin by mid-2008.
“The first F-35B is scheduled to begin its flight test program on time, with no delays foreseen,” John Kent, a Lockheed Martin spokesman, said in Dec. 20 e-mail. “The flight propulsion system (lift fan and engine) will be installed on schedule in early 2008.”
Since the JSF’s inception, there only have been two non-conforming rotors produced, which Gostic characterizes as “typical in a development program.”
“There is no design-related issue,” he said. “It basically arises from the fact that we need to fine-tune the manufacturing process with regards to [the] machining of those fan blades in that first stage rotor.”
The current plan is to remove the lift fan rotor in the first STOVL test jet in February and replace it with a dimensionally conforming rotor, which has already been built.
The Marine Corps will be the primary operator of the F-35B, however, the Air Force has not entirely ruled out the possibility of purchasing the STOVL variant.
Anticipating possible delays of the STOVL variant in fiscal year 2008, the Navy and Marine Corps in August 2006 cut funding from their portion of the Joint Strike Fighter program in their FY-08 to FY-13 spending proposal to factor in a 14-month slip, which would have pushed the purchase of 35 aircraft beyond 2013.
Deputy Defense Secretary Gordon England later reversed this cut, restoring funding and keeping the program on its current schedule.
The lift fan generates a column of cool air that provides nearly 20,000 pounds of power using “variable inlet guide vanes” to modulate the airflow, along with an equivalent amount of thrust from aircraft exhaust, according to an Air Force fact sheet. The fan utilizes a clutch that engages the shaft drive system for STOVL operations.
“Because the lift fan extracts power from the engine, exhaust temperatures are reduced by about 200 degrees compared to traditional STOVL systems,” the document notes.
On Dec. 18, Lockheed Martin executives hosted a ceremony at the aircraft assembly line in Ft. Worth, TX, to roll out the F-35B, attended by Marine Corps Commandant Gen. James Conway as well as foreign military representatives. The press release from the event quotes senior officials extolling the promise of the new aircraft, its STOVL capabilities in particular. No mention of the difficulties with the lift fan was noted.
The Air Force version of the stealth fighter successfully flew four times in December after a seven-month hiatus from flight testing, due to engine and electronic issues. It is unclear how these delays will impact the overall program schedule, Limrick said.
“The plane was ready to go on time, each time scheduled this month,” she said of the December flight tests.
Engine turbine design changes are being worked that would be complete by late 2008, she said.
“The F135 engine has been exceptionally reliable during the 23 flights conducted in the F-35’s first year of flight testing, and F135 engines have completed more than 8,500 hours of ground testing,” Kent said.
During engine ground testing last year, an issue with the third-stage turbine blade arose. This issue has been resolved in the short term, he said.
“In the meantime, we will proof test engines to validate turbine durability,” Limrick said. “A proof test will be performed on each flight test engine to ensure that the engines are not susceptible to the issue experienced on the 3rd stage turbine of a prior test engine.” -- Marcus Weisgerber and Jason Simpson
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